1. Technical Field
The present invention relates to the field of metrology, and more particularly, to accuracy estimation and improvement in imaging overlay targets.
2. Discussion of Related Art
FIG. 1 is a schematic illustration of an overlay target 90, comprising multiple periodic structures and a derivation of its center of symmetry, according to the prior art. The illustrated target comprises periodic structures at a previous (lower) layer and at a current (upper) layer, for which the centers of symmetry are derived by defining respective regions of interest (ROIs) 91, 92 in the optical image of the periodic structures and deriving kernels (projections, or intensity graphs, illustrated schematically within ROIs 91, 92) which are analyzed in pairs to determine their respective centers of symmetry of each layer, using the convolution expressed in Equation 1, with KL and KR representing the kernels in the pair:
                    ∫                  d          ⁢                                          ⁢                      x            ⁡                          (                                                                    K                    L                                    ⁡                                      (                                          x                      -                                              δ                        2                                                              )                                                  ×                                                      K                    R                                    ⁡                                      (                                                                  δ                        2                                            -                      x                                        )                                                              )                                                          Equation        ⁢                                  ⁢        1            
The quality of the target itself is currently assessed manually, e.g., inspected by eye, or indirectly through its impact on the measurement accuracy, using metrological accuracy merits.